1. Field of Invention
This invention relates to methods for manufacturing compacted graphite cast iron with a uniform distribution of vermicular or compacted graphite in thin walled shaped castings involving a range of cross sectional sizes. More particularly, this invention relates to continuous methods for manufacturing shaped castings, which have a uniformly distributed vermicular or compacted graphite morphology throughout the casting. Still more particularly, this invention relates to continuous, automated, high productivity methods for producing shaped castings of compacted graphite iron. The methods can be readily adopted by foundries presently equipped with high productivity, automated lines for production of ductile iron castings by nodularizing treatment inside the mold. The invention can be used for producing compacted graphite iron castings intermittently in a production line that is otherwise automated to produce ductile iron castings.
2. Description of the Prior Art
High productivity automotive lines have adopted a nodularizing treatment inside the mold, referred to hereafter as the "in-mold process", because the in-mold process better lends itself to automation than previous processes that involved batch treatment (i.e., sandwich or plunge processes). In the in-mold process, measured quantities of a graded magnesium ferrosilicon treatment alloy are introduced into a reaction chamber in the mold and the liquid iron to be treated is directed to flow over the treatment alloy in the reaction chamber. The incoming metal, having a well-controlled chemistry reacts with the magnesium ferro-silicon alloy in the mold resulting in a spheroidal graphite structure which imparts ductility to the cast iron. Descriptions of the in-mold technique can be found in U.S. Pat. Nos. 3,703,922, 3,819,365, 4,004,630 and 4,134,757 and British patents Nos. 743,121, 1,132,055 and 1,132,056.
During the in-mold process, the turbulence generated by magnesium vapor is used to advantage to ensure good mixing of the alloy with the melt. Also, the ferrosilicon serves not only as a convenient carrier of magnesium, but in addition inoculates the melt, producing the required number of graphite nuclei and thereby suppressing eutectic carbide formation. By carrying out the magnesium treatment inside the mold, good recovery of magnesium is ensured with minimal pollution problems. Further, the degeneration of graphite spherulitic structure caused by reoxidation is minimized. Also, a high nodule count is ensured, as there is no holding time involved for fading to occur. In comparison, in a batch treatment, the holding duration increases from the first casting poured till the last one, and consequently the quality of the castings within a batch varies. With the continuous in-mold process, on the other hand, this inherent variation of the batch process is eliminated.
An automated ductile line in a high productivity foundry typically consists of:
(i) A dispensing system for introducing measured quantities of the magnesium ferro-silicon treatment alloy into a pocket in the mold prior to the closing of the top half of the mold (i.e., cope). PA1 (ii) A pouring ladle system in which a measured quantity of liquid metal is received into a ladle attached to the end of a radial arm, which swings into position and locks onto a traversing mold on a conveyor system. Liquid metal from the ladle is dispensed into the pouring basin of the mold, once the ladle locks into position. PA1 (iii) After dispensing the liquid metal into the pouring basin of the mold, the arm swings through an idle cycle, before it locks into position at a metal receiving station. Typically, a number of radial arms are mounted on a central pivot, and the rate of pouring is varied to match the rate of production of the mold. PA1 (i) forming a near eutectic melt of cast iron having the chemistry of a ductile base metal for use in the in-mold treatment and having a low sulfur content, for example, about 0.01% by weight; PA1 (ii) adding to the melt sufficient graphite stabilizing agent, e.g., silicon, to suppress the carbide eutectic formation at large undercoolings characteristic of thinner sections of the casting; PA1 (iii) admixing at least one rare earth containing additive with said melt, e.g., by introducing a measured quantity of the additive into the pouring ladle just prior to tapping of the metal, the quantity of the additive being computed on the basis of reducing and maintaining Henrian activity of residual oxygen within the range of about 10.sup.-5 and 10.sup.-7 or more preferably within the range of about 10.sup.-5.5 and 10.sup.-6.5, or most preferably to about 10.sup.-6 ; PA1 (iv) tapping a predetermined quantity of liquid metal into the pouring ladle containing the additive; and PA1 (v) inoculating the melt and thereby provide the required degree of interconnected vermicular graphite growth. As the control of the required degree of nucleation is a critical step, this process step is preferably conducted by inoculating each mold individually just prior to casting, just as the molten metal enters the mold or inside the mold, for example in the molten metal path at the pouring basin, sprue or at a location between the sprue and ingate such as the nodularizing cavity or in-mold reaction chamber.
The ease of operation of the high productivity automated line for the manufacture of ductile iron castings having a spheroidal graphite structure has led to attempts to develop an in-mold alloy suitable for the production of iron castings having a compacted graphite structure. Since the turbulence created by magnesium vapor has been considered essential for the homogeneous mixing of the treatment alloy, the efforts to produce compacted graphite iron heretofore have focused on the development of an in-mold alloy based on magnesium.
Because of the narrow critical range (i.e., narrow window) within which magnesium is effective to provide compacted graphite morphology, the control of compacted graphite iron technology based on magnesium-containing in-mold alloys has proved formidable. Too little magnesium does not produce full compacted graphite structure; while over-treatment produces nodular graphite. The difference between under- and over-treatment can be as little as 0.005% by weight of magnesium.
Wider latitude or tolerance for magnesium has been obtained by using magnesium in conjunction with titanium to suppress the formation of nodular graphite. In such cases, however, a further complication is encountered in the formation of additional inclusions of titanium carbo-nitrides. See, for example, Schelleng, U.S. Pat. No. 3,421,886. Moreover, residual titanium in the recycled scrap will prove detrimental to the development of fully nodular graphite and thus impair the physical properties of ductile iron castings produced from the melt contaminated with titanium. In practice, it is not possible in an automated foundry to segregate scrap containing residual titanium arising from compacted graphite iron castings produced by magnesium-titanium technology, from those of ductile iron return scrap. Therefore, the process route for compacted graphite iron based on magnesium-titanium alloys is not favored in foundries that predominantly produce ductile iron castings lest there should be contamination of ductile iron castings by titanium.
Efforts to design treatment alloys based on a fixed ratio of magnesium to cerium or rare earths, in combination with inoculants, by empirical methods have not, as yet, yielded consistent results because of the difficulty in designing an optimal alloy to achieve the narrow window of magnesium required by compacted graphite, under the operating conditions in the field.
A recent patent, U.S. Pat. No. 4,396,428, is directed to the development of a low silicon, but magnesium-bearing iron alloy in order to establish a ready supply of treated molten iron in the holding vessels with a selected composition at a given temperature. However, the control of compacted graphite morphology warrants that the residual magnesium should be controlled within a narrow window and therefore the technology based on magnesium is inherently difficult to control. For the same reason, efforts to design treatment processes based on the injection of molten additives containing magnesium under high kinetic energy as in U.S. Pat. No. 4,227,924 are rendered difficult, in the case of compacted graphite iron.
Subramanian et al have identified the broader window offered by rare earths to control the impurity concentrations in the cast iron melt for the consistent production of compacted graphite morphology in thick sections of tonnage castings, see U.S. Pat. No. 4,227,924. For instance the model test blocks used in examples 2-6 cited in the patent are of size 15".times.15".times.8". At slow cooling rates characteristic of such thick sections, there is negligible kinetic undercooling, and therefore the freezing occurs at near equilibrium conditions. At such low undercoolings typical of thick sections, the competitive growth of the carbide (cementite) phase does not occur. Accordingly, inoculation is not cited as an essential step to control the compacted graphite morphology. Further, at such low undercoolings involving smaller driving forces for graphite growth, the impurity dependent crystal growth mechanism that favors compacted graphite growth dominates over spherulitic graphite growth, thereby minimizing the nodularity problem in compacted graphite structure. Thus, in the absence of both the carbide problem and the nodularity problem at the small undercoolings operating in thick sections, the production of compacted graphite morphology is determined by controlling the impurity concentrations in the melt, to correspond to a Henrian sulfur activity window between 0.004 and 0.035, the upper and lower bounds of the Henrian oxygen activity window being 10.sup.-b 4 and 10.sup.-6.
In a thin walled shaped casting involving a range of cross sectional sizes, a range of cooling rates and therefore, a range of kinetic undercoolings are involved; the thinner the section size, the larger the kinetic undercooling or the deviation from equilibrium freezing. Consequently, the structure varies as a function of cross sectional size. Thus, thinner sections freeze with large kinetic undercooling, leading to significant deviation from equilibrium freezing, in marked contrast to thicker sections that freeze under near equilibrium conditions.
Under the conditions of large kinetic undercoolings that characterize the freezing of thinner cross sections, competitive growth of cementite dominates over graphite growth, resulting in the formation of hard and brittle carbide eutectic structure. Further, under conditions of large kinetic undercoolings, the spiral growth on the basal face of graphite that promotes spherulitic morphology dominates over impurity dependent crystal growth mechanisms that promote prism flake growth also described hereinbelow. Consequently the tendency for nodularity increases as the cross section size decreases. The graphite morphological variation as a function of section size in a casting is referred to as section sensitivity of the casting. In FIG. 1 described hereinbelow, the design of a finned casting used in the section sensitivity test is illustrated; FIG. 2, also described hereinbelow, shows a typical increase in the degree of nodularity as the section size decreases, in a melt treated with cerium to produce compacted graphite morphology in thicker sections and inoculated with ferrosilicon just prior to casting. On holding the melt after inoculation prior to casting, carbides develop in thinner sections of the fins. Thus, compacted graphite morphology control in thinner sections of a shaped casting is inherently more difficult than in thicker sections because the thinner sections freeze with a large kinetic undercooling that deviates significantly from equilibrium freezing conditions, and are therefore prone to a greater tendency toward carbide formation and an increased degree of nodularity.